1. Field of the Invention
The present invention relates to an advanced grid structure which has high strength and low thermal expansion characteristics, and which is formed by using carbon fiber reinforced plastic that is a material for use in the aerospace which is lighter than metal and has a low thermal expansion coefficient.
2. Description of the Related Art
In recent years, along with increase in demand for high resolution images of the Earth's surface, there is made a plan to arrange a plurality of small satellites incorporating optical devices in the low earth orbit. Accordingly, an importance is put on development of the small satellites incorporating the optical devices. In order to prevent reduction in observation accuracy of the optical devices, there is a demand for each of the satellites to have a satellite structure having thermal dimensional stability. Further, unlike medium and large satellite structures which need to have high rigidity, the small satellite structures need to have high strength.
As the satellite structure having thermal dimensional stability, there is suggested a low thermal expansion structure having a complex quadrangular lattice. The low thermal expansion structure of the complex quadrangular lattice is obtained by combining quadrangular tubes and rods having slots. The  quadrangular tubes are formed by utilizing characteristics differing in thermal expansion coefficient between a direction in which carbon fibers of the carbon fiber reinforced plastic run and a direction perpendicular to the direction in which the carbon fibers run, and by adjusting an orientation angle of the carbon fibers so that the thermal expansion coefficient becomes close to zero. Further, the rods include first rods made of the carbon fiber reinforced plastic and second rods made of the carbon fiber reinforced plastic. Slits are provided to those in positions which fit with each other. The low thermal expansion structure is assembled by arranging the first rods and the second rods perpendicularly to each other and fitting those by the slits and is structured by fitting the quadrangular tubes to an inner side surrounded by side surfaces of the rods. With this structure, a structure having a value of the thermal expansion coefficient close to zero is realized (see, for example, authors, K. J. Yoon and three others “COMPOSITE GRID STRUCTURE WITH NEAR-ZERO THERMALLY INDUCED DEFLECTION”, 41st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference and Exhibit, April 2000, AIAA-200-1476, pp. 971-976).
However, in the above-mentioned low thermal expansion structure, while the thermal expansion coefficient can be set within a range from −1.0 ppm/K to 1.0 ppm/K, the rods are fitted and adhered to each other through the slits. Accordingly, there is a problem in that strength of the low thermal expansion structure is limited due to a low tensile strength of about 40 MPa at the adhesion portions, so the strength is reduced.
In this case, when a conventional pseudoisotropic laminated structure is  structured by using carbon fibers having a tensile modulus of elasticity of 280 GPa or more and 330 GPa or less, it is possible to realize the pseudoisotropic laminated structure having high strength, whose tensile strength is 4600 MPa or more, but whose thermal expansion coefficient is 1.1 ppm/K or more.
In general, carbon fibers have a negative thermal expansion coefficient, and a resin has a positive thermal expansion coefficient. Accordingly, while the carbon fiber reinforced plastic having a one-direction laminated structure of the carbon fibers has the negative thermal expansion coefficient in a direction of the carbon fibers, the pseudoisotropic laminated structure formed thereby has the positive thermal expansion coefficient.
Further, the modulus of elasticity and the thermal expansion coefficient of the carbon fibers have an inverse proportional relationship, and the modulus of elasticity and the strength thereof also have an inverse proportional relationship. As a result, the strength and the thermal expansion coefficient of the carbon fibers have a proportional relationship. Accordingly, in the pseudoisotropic laminated structure formed by using the carbon fibers having high strength, a thermal expansion coefficient cannot be made close to zero. Therefore, there is a problem in that this pseudoisotropic laminated structure is not appropriate for the satellite structure incorporating optical devices in terms of the thermal dimensional stability.
Further, conventional earth observatory satellites have medium or large sizes, so there are needs for satellite structures having high rigidity and low thermal expansion characteristics. Accordingly, there is not reached an object of developing a satellite structure having high strength and low thermal  expansion characteristics.